Conventional optical fiber cables have a multi-layer construction. At the center of an optical fiber cable is a transparent core. Typically made of glass, a single mode optical core has a diameter in the range of about 5-10 microns (.mu.m). Multimode mode cores can be substantially larger. The optical core is surrounded by a concentric cladding, usually glass, that has an outer diameter that is typically about 125 .mu.m. The cladding has a refractive index that is lower than the refractive index of the core. As a result, light energy is confined to the core region. The cladding, and encompassed optical core, are often referred to as "optical fiber," or "bare optical fiber."
For telecommunication applications, the bare optical fiber is covered by a protective coating. More specifically, the bare optical fiber is typically coated with a relatively soft inner layer, which is then overcoated with a hard external shell. Both of the layers are typically methacrylates characterized by different values of Young's Modulus. Such a dual coating provides a measure of strain relief and reduces the incidence of fiber breakage. The inner layer acts as a soft buffer to minimize microbending, while the outer layer functions as a protective shell. For a 125 micron diameter bare optical fiber, the inner coating layer is typically about 30 to 40 microns radial thickness. The outside diameter of the outer coating layer is about 250 microns.
In unprotected environments, one or more bare optical fibers are carried within a protective cable. The strength, complexity and cost of the cable depends on the environment. For example, an office environment "break out" cable consists of a plastic outer jacket, a layer of loose kevlar fiber for strain relief and padding, and a 900 micron diameter tight polyester buffer in contact with the bare fiber. In simplex cable, which carries a single optical fiber, the optical fiber itself represents only a small fraction of the total cost. Normal duplex cable, which consists of two simplex cables joined by a thin plastic bridge, is double the volume and costs about 75 percent more than simplex cable.
The cost of installing optical fiber can be large compared to the cost of the fiber itself. Therefore, when optical fiber is installed, such as within a building or between buildings, it is common to layer more fiber than is actually required for the intended application. This additional fiber is called dark fiber, because it does not initially carry an optical signal. The dark fiber might never be used. If, however, more lines are required at a later date, the dark fiber can be connected and used. Such an approach is cost effective because it avoids repeating the large expense of fiber installation.
Simplex fiber optic links are made with one fiber for both transmitted and received signals. Duplex fiber optic links, which use a separate fiber for the transmitted and received signals, offer advantages in signal intensity and noise levels. A network installation originally intended for simplex links could later be converted into a duplex network provided sufficient dark fiber is included during the initial installation. An optical fiber installation with more than double the initial fiber link requirements is therefore amenable to such an upgrade. Doubling the fiber count in an installation would, however, represent a significant increase in total cost.
As such, it would be desirable to provide a duplex optical fiber that lowers the cost of a duplex fiber link and reduces the cost associated with providing sufficient dark fiber for upgrading a simplex link to a duplex link.